:root {
  --bg: #f3f4f7;
  --paper: #ffffff;
  --ink: #1f2330;
  --muted: #5a6175;
  --accent: #3458d6;
  --accent-dark: #1f3a9a;
  --border: #ced3df;
  --warning: #d8463a;
  --shadow: 0 4px 12px rgba(20, 30, 60, 0.08);
}

* { box-sizing: border-box; }

html, body { margin: 0; padding: 0; }

body {
  font-family: -apple-system, BlinkMacSystemFont, "Segoe UI", Roboto, "Helvetica Neue", Arial, sans-serif;
  color: var(--ink);
  background: var(--bg);
  line-height: 1.55;
  font-size: 16px;
}

main { max-width: 1080px; margin: 0 auto; padding: 0 20px 60px; }

.skip-link {
  position: absolute; top: -100px; left: 12px;
  background: var(--ink); color: var(--paper);
  padding: 10px 14px; border-radius: 6px;
  font-weight: 700; text-decoration: none;
  z-index: 100;
}
.skip-link:focus { top: 12px; outline: 3px solid var(--accent); }

#main { outline: none; }

:where(a, button, summary, [tabindex]):focus-visible {
  outline: 3px solid var(--accent); outline-offset: 2px; border-radius: 4px;
}
.button:focus-visible { outline-offset: 3px; }

@media (prefers-reduced-motion: reduce) {
  * { animation-duration: 0s !important; transition-duration: 0s !important; }
}

.site-header {
  display: flex; align-items: center; justify-content: space-between;
  padding: 14px 28px; background: var(--paper);
  border-bottom: 1px solid var(--border);
  position: sticky; top: 0; z-index: 10;
}
.brand {
  display: inline-flex; align-items: center; gap: 10px;
  text-decoration: none; color: var(--ink); font-weight: 700;
}
.brand-mark {
  display: inline-block; width: 18px; height: 18px;
  background:
    linear-gradient(45deg, #d8463a 25%, transparent 25%, transparent 50%, #ffe14a 50%, #ffe14a 75%, transparent 75%);
  background-size: 18px 18px;
  border: 1px solid #222;
}
.site-header nav { display: flex; gap: 14px; }
.site-header nav a {
  color: var(--muted); text-decoration: none; font-size: 0.95rem;
}
.site-header nav a:hover { color: var(--accent-dark); }

.hero {
  padding: 36px 0 24px;
  border-bottom: 1px solid var(--border);
}
.hero.utility-layout {
  display: grid; gap: 32px; grid-template-columns: minmax(0, 1.6fr) minmax(0, 1fr);
  align-items: start;
}
.hero.simple-index { grid-template-columns: 1fr; }
.hero h1 { font-size: 2.0rem; margin: 6px 0 14px; line-height: 1.2; }
.hero .lede { font-size: 1.05rem; color: var(--muted); margin: 0 0 12px; }
.hero .answer {
  background: var(--paper); border: 1px solid var(--border); border-radius: 10px;
  padding: 12px 16px; box-shadow: var(--shadow);
}
.eyebrow {
  font-size: 0.78rem; text-transform: uppercase; letter-spacing: 0.08em;
  color: var(--accent-dark); margin: 0 0 6px;
}

.meta-strip {
  display: grid; gap: 10px;
  grid-template-columns: repeat(auto-fit, minmax(150px, 1fr));
  margin-top: 18px;
}
.meta-strip > span {
  background: var(--paper); border: 1px solid var(--border); border-radius: 8px;
  padding: 8px 10px; display: flex; flex-direction: column;
}
.meta-strip em {
  font-style: normal; font-size: 0.72rem; color: var(--muted);
  text-transform: uppercase; letter-spacing: 0.06em;
}
.meta-strip strong { font-size: 0.95rem; color: var(--ink); }

.fold-summary {
  background: var(--paper); border: 1px solid var(--border); border-radius: 12px;
  padding: 16px 18px; box-shadow: var(--shadow);
}
.fold-summary h2 { margin: 4px 0 12px; font-size: 1.1rem; }
.small-note { color: var(--muted); font-size: 0.82rem; margin-top: 10px; }

.result-grid {
  display: grid; gap: 8px; grid-template-columns: repeat(2, 1fr);
}
.result-grid.compact .result-metric {
  background: var(--bg); border: 1px solid var(--border); border-radius: 8px;
  padding: 8px 10px; display: flex; flex-direction: column;
}
.result-grid.compact .result-metric.accent { background: var(--accent); color: white; }
.result-grid.compact .result-metric.accent span,
.result-grid.compact .result-metric.accent strong { color: white; }
.result-grid.compact .result-metric span {
  font-size: 0.72rem; color: var(--muted);
  text-transform: uppercase; letter-spacing: 0.06em;
}
.result-grid.compact .result-metric strong { font-size: 1.2rem; }

.band { padding: 32px 0; border-bottom: 1px solid var(--border); }
.band.split { display: grid; gap: 28px; grid-template-columns: minmax(0, 1.4fr) minmax(0, 1fr); align-items: start; }
.section-heading { margin-bottom: 16px; }
.section-heading h2 { margin: 4px 0 0; font-size: 1.5rem; }

.check-list { padding-left: 18px; }
.check-list li { margin-bottom: 6px; }

.formula-box {
  background: var(--paper); border: 1px solid var(--border); border-radius: 10px;
  padding: 16px; box-shadow: var(--shadow);
  display: flex; flex-direction: column; gap: 6px;
}
.formula-box strong { font-size: 1.2rem; color: var(--accent-dark); }
.formula-box span { color: var(--muted); font-size: 0.9rem; }

.button {
  display: inline-block; padding: 10px 18px; border-radius: 999px;
  font-weight: 600; text-decoration: none; cursor: pointer;
  border: 1px solid var(--border); background: var(--paper); color: var(--ink);
  font-size: 0.95rem;
}
.button.primary { background: var(--accent); color: white; border-color: var(--accent-dark); }
.button.primary:hover { background: var(--accent-dark); }
.button:hover { background: var(--bg); }
.button.ghost { background: transparent; }
.button-row { display: flex; flex-wrap: wrap; gap: 10px; margin-top: 8px; }

.notation-strip {
  display: grid; gap: 8px;
  grid-template-columns: repeat(auto-fit, minmax(220px, 1fr));
}
.notation-item {
  background: var(--paper); border: 1px solid var(--border); border-radius: 8px;
  padding: 8px 12px; display: flex; gap: 12px; align-items: baseline;
}
.notation-item code {
  background: var(--bg); border-radius: 4px; padding: 2px 8px;
  font-weight: 700; color: var(--accent-dark); min-width: 36px; text-align: center;
}
.notation-item span { color: var(--ink); font-size: 0.9rem; }

.fold-mode {
  background: var(--paper); border: 1px solid var(--border); border-radius: 12px;
  padding: 18px; box-shadow: var(--shadow);
}
.fold-controls {
  display: flex; flex-wrap: wrap; gap: 10px; align-items: center;
  padding-bottom: 14px; margin-bottom: 14px; border-bottom: 1px solid var(--border);
}
.fold-progress { font-weight: 700; color: var(--accent-dark); }
.fold-steps { list-style: none; margin: 0; padding: 0; }
.fold-step {
  display: none; padding: 14px 0;
}
.fold-step.is-active { display: block; }
.fold-step-head {
  display: flex; gap: 12px; align-items: baseline; flex-wrap: wrap;
  margin-bottom: 10px;
}
.fold-step-head .step-number { font-weight: 700; color: var(--accent-dark); font-size: 1.05rem; }
.fold-step-head .step-of { color: var(--muted); font-size: 0.85rem; }
.fold-step-head .step-title { color: var(--ink); font-weight: 600; font-size: 1.05rem; }
.fold-step-body { display: grid; gap: 18px; grid-template-columns: 180px 1fr; align-items: start; }
.fold-diagram {
  background: var(--bg); border: 1px solid var(--border); border-radius: 8px;
  padding: 10px;
}
.fold-diagram .diagram-svg { width: 100%; height: auto; max-height: 180px; }
.fold-step-text p { margin: 0 0 10px; }

.mistake-callout {
  background: rgba(216, 70, 58, 0.06); border-left: 3px solid var(--warning); border-radius: 4px;
  padding: 8px 12px; color: var(--ink); font-size: 0.95rem;
}
.mistake-callout summary {
  cursor: pointer; font-weight: 600; color: var(--warning);
  list-style: revert;
}
.mistake-callout p { margin: 10px 0 0; }

.algorithm-grid {
  display: grid; gap: 12px;
  grid-template-columns: repeat(auto-fit, minmax(260px, 1fr));
}
.algorithm-item {
  background: var(--paper); border: 1px solid var(--border); border-radius: 10px;
  padding: 14px 16px; box-shadow: var(--shadow);
  display: flex; flex-direction: column; gap: 6px;
}
.algorithm-item .eyebrow { margin-bottom: 0; }
.algorithm-notation {
  background: var(--ink); color: #ffe14a;
  padding: 8px 10px; border-radius: 6px;
  font-family: ui-monospace, "SF Mono", Menlo, Consolas, monospace;
  font-size: 1.0rem; font-weight: 600;
  align-self: flex-start;
}
.algorithm-item p { margin: 0; color: var(--ink); font-size: 0.9rem; }

.algorithm-grid.printable { gap: 8px; }
.algorithm-grid.printable .algorithm-item { box-shadow: none; }

.related-grid {
  display: grid; gap: 12px;
  grid-template-columns: repeat(auto-fit, minmax(230px, 1fr));
}
.related-grid a {
  text-decoration: none; color: var(--ink);
  background: var(--paper); border: 1px solid var(--border); border-radius: 10px;
  padding: 14px 16px; display: flex; flex-direction: column; gap: 4px;
  box-shadow: var(--shadow);
}
.related-grid a:hover { border-color: var(--accent); }
.related-grid a span {
  font-size: 0.72rem; color: var(--accent-dark);
  text-transform: uppercase; letter-spacing: 0.06em;
}
.related-grid a strong { font-size: 1.0rem; }
.related-grid a p { margin: 4px 0 0; color: var(--muted); font-size: 0.88rem; }

.site-footer {
  padding: 26px 20px; color: var(--muted); font-size: 0.88rem;
  border-top: 1px solid var(--border); text-align: center;
}
.site-footer .footer-meta { margin-top: 6px; }
.site-footer .footer-meta a {
  color: var(--accent-dark); text-decoration: none; margin: 0 6px;
}
.site-footer .footer-meta a:hover { text-decoration: underline; }

.banner-noindex {
  background: rgba(216, 70, 58, 0.12);
  border: 1px solid var(--warning);
  color: var(--ink);
  padding: 10px 14px; border-radius: 8px; margin: 16px 0;
  font-size: 0.92rem;
}
.banner-noindex strong { color: var(--warning); }

.prose-page { max-width: 720px; }
.prose-page h2 { margin-top: 24px; }
.prose-page p, .prose-page li { color: var(--ink); }
.prose-page p { margin: 12px 0; }
.prose-page ul { padding-left: 20px; }
.prose-page ul li { margin-bottom: 6px; }
.prose-page code { background: var(--bg); padding: 2px 5px; border-radius: 4px; }

@media (max-width: 820px) {
  .hero.utility-layout, .band.split { grid-template-columns: 1fr; }
  .fold-step-body { grid-template-columns: 1fr; }
}

@media print {
  .site-header, .site-footer, .fold-controls, .related-grid, .no-print { display: none !important; }
  .fold-step { display: block !important; page-break-inside: avoid; }
  body { background: white; color: black; }
  .band { border-bottom: none; padding: 12px 0; }
  .hero { border-bottom: 1px solid #ccc; padding: 16px 0; }
  .fold-mode, .algorithm-item, .notation-item, .formula-box, .fold-summary, .meta-strip > span {
    box-shadow: none;
  }
  .algorithm-notation { background: #222; color: #ffe14a; }
  .mistake-callout { background: #f9f9f9; }
}